1. Field of the Invention
The present invention relates to an apparatus for superplastic forming for forming a metal sheet or plate by compressing it with a gas.
2. Description of the Background Art
Superplastic forming is a technology of forming a metallic sheet or plate mounted on a die in a sealed chamber by compressing it with a gas from opposite side of the die using a property that a material of fine grain structure has very high ductility of hundredsxcx9cthousands % and low flow stress in a range of strain rate and high temperatures. Therefore, the forming method has advantages such that deep and complex shapes, which can not be formed by another forming methods such as a conventional deep drawing method, can be formed in one piece in a single press cycle.
However, the conventional superplastic forming method has a problem to fabricate massively a lot of products with one press, because the forming process requires heating of dies and sheet metals to a uniform high temperature, and is conducted at low speed with one or a few dies arranged on a single bed of a same floor. Therefore, in order to produce massively in the conventional method, a lot of forming presses should be made, or a big size of the bed should be used so that a lot of dies can be arranged simultaneously on a single apparatus.
However, the former conventional method has a disadvantage that the cost of the product increases since fabrication of a lot of forming apparatuses needs high cost. Also, the latter conventional method has disadvantages such that an expensive high-power press should be used since the force of the press to keep chambers or dies closed during forming increases in proportion to the total area of the horizontal plane of dies on the bed where forming pressure is applied (plane area of the bed), and thereby, the fabrication cost is also increased.
Furthermore, since the superplastic forming is performed at a low speed, cycle times for forming range from 10 to 100 minutes or more according to depth of component and detailed design. Therefore, the superplastic forming can not be applied to the fields requiring mass production such as car industry, and has been used in the fields of small quantity batch production such as aerospace industry, railway vehicle, sports car, medical instrument, military electric device and architecture panels so far. Moreover, as the superplastic forming is done mostly under a plane strain or biaxial tension state having low forming limit, there is a limit in the depth which can be formed. Thus, an expensive material having extra high ductility should be used in order to make a product of high aspect ratio (ratio of a depth to a width of the product). Also, thickness distribution is not uniform after the superplastic forming, and therefore, complicated methods and processes should be used in case that uniform thickness is required.
An object of the present invention is to provide a superplastic forming apparatus of a new structure by which mass production can be made without increasing forming apparatuses or using a high-power press with a big size bed.
Another object of the present invention is to provide a forming method by which a product having a large aspect ratio and uniform thickness can be formed with high productivity using an ordinary superplastic material, and without using a special apparatus and material of high price.
To achieve the objects of the present invention, as embodied and broadly described herein, the present invention arranges chambers and dies in up-and-down direction of a press instead of arranging them horizontally on a wide bed in a high-power press or a large number of presses, in order to produce massively the products without using a high-power press having a large size bed or without increasing the number of the presses.
Also, to achieve another object of the present invention mentioned above, a superplastic forming method is combined with a deep drawing process, where a simple shape of high aspect ratio is made in advance by a deep drawing method, and used as a preform for subsequent superplastic forming to complete a complex is shape. By this method, a complex shape having high aspect ratio can be made with some additional advantages of a uniform thickness distribution and a short total forming time in a minute or less, since deep drawing leads to more uniform thickness distribution than superplastic forming, and it takes only a few seconds to make the preform by deep drawing, and tens of seconds to complete the remaining complex shape by superplastic forming. Otherwise, it takes tens of minutes or more to form a complex shape by the conventional superplastic forming method.
In detail, to achieve the objects of the present invention, there is provided an apparatus for superplastic forming, which allows a large number of complex parts to be formed from sheets in a press by forcing heated sheet into dies arranged on several layers with compressed gas or air. The apparatus consists of several sets of vertically layered dies having one or more gas injection holes and one or more gas discharging holes; a heating unit for providing dies with heat; a sets of sealed chambers wherein dies are installed in a pressurized state during forming; and a press for compressing dies or chambers. According to circumstances, the sealed chamber may not be used, and then, the die itself may have the functions of the sealed structure.
The gas injection hole may be formed on the upper metallic pattern, on the lower metallic pattern, or may be formed on intermediate portion of the upper and lower metallic patterns.
Two sheets of forming plates can be inserted into the respective dies in each layer, and in this case, it is desirable that the gas injection hole is formed so that the gas can be injected between the two plates.
The apparatus of the present invention may further comprises an auxiliary device between the upper and the lower dies. In this case, the gas injection hole may be formed in the auxiliary device, and a pair of forming sheets to be formed simultaneously is inserted into the upper and lower parts of the auxiliary device.
In another embodiment of the present invention, chambers and dies are installed on plural floors or layers, and the respective chambers or dies includes one or more guide rods which move in the guide hole of the opposite chambers or dies. The respective chambers or dies are guided by the guide rod, and thereby, can be separated each other or coupled together without misfit or dislocation.
Also, another embodiment of the present invention further comprises a supporting board adjacent to both sides of the sealed structure in vertical direction. In this case, the chamber on each layer includes one ore more protruded portions on outer surface thereof, and one or more recesses indented toward inside of the supporting board surface are formed on the positions corresponding to the protruded portion. The protruded parts and indented recesses which are matched together as a pair are used to separate or couple the adjacent layers one by one or simultaneously.
For successive separation or coupling, the respective recesses have different lengths from those of each other, and it is desirable that the length of lower recess is longer than that of an upper guide recess in order to lift the upper layer first and then the lower layer next. On the other hand, the recesses have the same length for simultaneous separation or coupling.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.